Logarithmic circuit element



Nov. 19, 1963 J. PRAGLIN 3,111,627

LOGARITWIC CIRCUIT ELEMENT Original Filed May 2, 1958 INVENTOR. Jam/sP/aAeL/N nited States Patent Ofifice 3-,l l L627 Patented Nov. 19, 1963The present invention relates to circuits, such as micromicroammeters,utilizing a vacuum tube as a logarithmic device to provide a voltageacross a pair of elements thereof which varies as the logarithm or" thecurrent between the elements. This application is a continuation of myapplication Serial No. 732,708, filed May 2, 1958, which applicationbecame abandoned on October 14, 1961.

Vacuum tubes have been used as logarithmic circuit devices to provide avoltage which varies as a logarithrnic function of a current in thevacuum tube. Diodes are commonly used as logarithmic circuit devices butthey have not proved entirely satisfactory since the commercial diodeshave a relatively high leakage resistance and, furthermore, from a givengroup of commercial diodes it is not uncommon to find that only of thediodes satisfactory for use as logarithmic elements.

Other vacuum tubes, such as an electrorueter tube having a control grid,have been utilized as logarithmic circuit devices to provide a varyingvoltage which is a function of the logarithm of the grid current of thetube. However, when such tubes have heretofore been connected to providea voltage which varies as the logarithmic function of the grid current,the range of response of the tube has been seriously affected. Forexample, the (SK-5886 electrometer tube which is outstanding when usedas a logarithmic element will only have a linear response for currentsin the micromicroammeter range which vary over about 4 /2 decades whenconnected in accordance with the prior art.

The principal object of the present invention is to provide a new andimproved circuit in which a vacuum tube, particularly an electrometertube in which the grid is highly insulated relative to the otherelectrodes, is utilized as a logarithmic diode device and is connectedso as to be accurately responsive over a wider range of input cur rentsthan possible with similar vacuum tube logarithmic circuit elements ofthe prior art.

A further object or" the present invention is to provide a new andimproved circuit including a vacuum tube which functions as alogarithmic diode circuit element to provide a voltage which varieslogarithmically with respect to a small input current to the tube and inwhich the response will be linear for currents as small as 19* amperes.

Another object of the present invention is to provide a new and improvedcircuit including a vacuum tube having a control grid and connected todevelop a voltage which varies logarithmically with a small grid currentin the tube and in which the plate is maintained positive with respectto the cathode and, preferably, with respect to the grid.

Further objects and advantages of the present invention will be apparentfrom the following detailed description of the preferred embodiment madewith reference to the accompanying drawings and in which the sole FIGUREis a schematic circuit diagram of a measur ng circuit including alogarithmic circu' element.

While the present inver on is susceptible of use in various circuitswhere it is desired to produce a voltage which varies logarithmicallywith a snail current, it is herein shown as embodied in a circuit formeasuring small input currents and which has an output voltage whichvaries as a logarithmic function of the input current.

Referring to the drawing, the small current to be measured is appliedbetween the input terminals 10, 11 of the measuring circuit. Themeasuring circuit includes a DC. amplifier 13, preferably, anelectrometer tube amplifier, having amplifier input terminals 14, 15.The amplifier 13 may be substantially the same as that shown anddescribed in an article entitled A New High StabilityMicromicroarnmeter, in IRE. Transactions on Instrumentation, volume 1-6,No. 2, pages 144-147 (1957). The amplifier input terminal 14 isconnected to the measuring circuit input terminal it? and the otheramplifier input terminal 15 is grounded, as is the other measuringcircuit input terminal 11. The amplifier has output terminals 17, it";across which a voltage dividing resistance 2b is connected as well as aseries circuit including a resistance and an indicating meter 21 havinga logarithmic scale.

The voltage dividing resistance 26 has a movable tap 22 and the tap 22is connected to the input of the amplifier so that a portion of thevoltage developed across the resistance 2% is degeneratively fed back tothe input of the amplifier.

The feed-back loop of the amplifier includes a logarithmic vacuum tube23, a tetrode in the illustrated embodimerit having a plate 24, a screengrid 25, a control grid 26, and a filamentary cathode or filament 27.The tube 23 is preferably an electrometer tube which has the gridthereof hi hly insulated from the other elements of the tube.Electrometer tubes are generally defined as tubes having a grid currentor" 16- amperes or less when the tube is operated as amplifier. Heatingcurrent for the filament 27 is supplied from a full wave rectifier 32.The filament 27 is connected to output terminals 30, 31 of the full waverectifier 32 by a resistor 33 connecting one side or" the filament tothe positive terminal 34 of the rectifier, the other side of thefilament being connected to the negative terminal of the rectifier by aresistance 3 t and a potentiometer-type resistance 35 connected inseries between the filament and the rectifier. The potentiometer-typeresistance 35 has a movable tap 36 connected to the movable tap 22 ofthe resistance Ztl. A filtering condenser 37 is preferably connectedacross the output of the full wave rectifier.

The control grid 26 of the tetrode is connected to the measuring circuitinput terminal it by a resistor 38. The grid as functions as the anodeof the tube and an input current applied to the terminal it will cause acurrent to flow between the control grid, functioning as an anode, andthe cathode or filament 27. For extremely small input currents, forexample, input currents of the order of magnitude of 10* amperes, thegrid-cathode voltage will vary logarithmically with respect to the inputcurrent over a range of decades depending on the tube utilized. Theinput voltage to the amplifier 13 and, in turn, the output thereofvaries in accordance with the grid-tocathode voltage of the tube 2.3,and, therefore, varies logarithmically with the input current to themeasuring circuit.

it has been discovered that the range of logarithmic response of thetube 23 is increased if the plate 24-, and the screen grid when atetrode is used as in the illustrated embodiment, is maintained positivewith respect to the filament and preferably with respect to the controlgrid. For example, it has been found that when a CK-5886 tube isconnected as a tetrode in the manner described, the tribe performs verysatisfactorily if the plate is about 1.5 to 3.0 volts positive withrespective to the filament. The plate 24 is connected to the positiveterminal 30 of the rectifier and the resistor 33 is chosen so as toprovide a desired voltage drop between the plate md the filament.

3 The screen grid is connected to the plate so as to be maintained atthe same potential as the plate.

The operation of the measuring circuit has not been described in detailsince the present invention relates to the manner of obtaining a voltagewhich varies logarith mically with respect to a small current. Sufliceit to say that in the illustrated embodiment the input current isestablished between the terminals 1%), 11 and flows in the controlgrid-filament circuit of the tube 23 to provide a voltage at the inputterminals of the amplifier 13 which varies logarithmically with theinput current. The grid 26 is biased by the voltage developed across theresistor 34 and the part of the resistance of potentiometer resist ance35 which is on the positive side of the tap 36. The biasing voltagebucks the input current applied to the terminals 10, 11. Thegrid-cathode circuit may be traced from the input terminals 10 and '11through the resistor 38, the grid 26, the cathode 27, the resistor 34,the upper portion of the resistance 35, the tap 36, the tap 22, thelower portion of the resistance 20 and the ground connections back tothe terminal 11.

In addition to the biasing Voltage, a portion of the output voltage isfed back by the connection between the tap 22 and the tap 36 and alsobucks the input current flowing in the control grid-cathode circuit ofthe tube 23.

While the logarithmic device has been described as connected in thefeed-back loop of the amplifier 13, it may be connected in shunt withthe input of the amplifier and, as mentioned above, it may be utilizedin other circuits where it is desirable to obtain a voltage which variesas the logarithm of a small input current.

In the preferred embodiment of the present invention, the vacuum tubeshown is a tetrode and the screen grid of the tetrode is connected tothe plate and maintained at the same potential to the plate. It will beunderstood that other tubes having at least a plate, control grid, andcahode, preferably a directly heated cathode, may be utilized as alogarithmic diode with the plate maintained positive with respect to thecathode and preferably the control grid as well and operated as apassive circuit element which is independent of circuits into which theelements of the tube forming the logarithmic diode are connected.

It can now be seen that the present invention provides a new andimproved circuit wherein a vacuum tube is used as a diode to provide avoltage which varies logarithmically with respect to a current betweenelements of the tube, the tube having a cathode, control grid, andplate, with the control grid and cathode constituting the principalelectrodes of the tube when connected into the circuit, the plate beingconnected into the circuit as an auxiliary electrode, i.e., as a passivecircuit element, and is maintained positive with respect to the cathodeand preferably the grid as well. The improved circuitry extends therange of responsive of the vacuum tube and enables the circuit to beused over a wider range of current.

While the preferred embodiment of the present invention has beendescribed in considerable detail, further constructions, arrangements,and modifications will appear to those skilled in the art, and it ishereby my intention to cover all such constructions, arrangements, andmodifications which fall within the spirit and scope of the presentinvention.

Having described my invention, what I claim is:

1. In an electrical circuit for providing a voltage which varieslogarithmically with variations of a small current and including avacuum tube functioning as a nonlinear circuit element and having aplate, a grid, and a cathode, input circuit means connected to thegrid-cathode path of saidtube to establish a variable grid current fromthe grid to the cathode, whereby said grid and cathode act as alogarithmic diode in which the voltage between said grid and cathodevaries as a logarithmic function of said current between said grid andcathode, output circuit means connected between said grid and cathodeand responsive to the giid-to-cathode voltage, and biasing circuit meansconnecting said plate and cathode and biasing said plate by directlyconnecting said plate to a substantially fixed potential which ispositive with respect to said cathode, said output circuit means beingconnected independently of said plate, said plate functioning as apassive circuit element.

2. In an electrical circuit, the combination as defined in claim 1,wherein said biasing circuit means biases said plate about 1.5 to about3 volts positive with respect to said cathode.

3. In an electrical circuit for providing a voltage which varieslogarithmically with variations of a small current and including avacuum tube functioning as a nonlinear circuit element and having aplate, a grid, and a directly heated cathode, input circuit meansconducting said small current to the grid-cathode path of said tube toestablish a. current between the grid and cathode, said grid and cathodedefining a logarithmic diode with the grid-cathode voltage varyinglogarithmically with the grid current from said grid to said cathode, adirect current power supply for heating said cathode, output circuitmeans connected between said grid and cathode for energization inaccordance with said grid-cathode voltage, an impedance connecting thepositive side of the power supply to said cathode, and means directlyconnecting said plate to the positive side of said impedance so that thepotential applied to the plate has a substantially fixed value, saidoutput circuit means being connected independently of said plate wherebysaid plate functions as a passive circuit element.

4. In an electrical circuit for providing a voltage which varieslogarithmically with variations of a small current including a vacuumtube functioning as a nonlinear circuit element and having a plate, ascreen grid, a control grid, and a directly heated filament constitutingthe cathode thereof, input circuit means connected to the grid-cathodepath of said tube to establish a grid current from grid to said cathode,said grid and cathode defining a logarithmic diode with the grid-cathodevoltage varying logarithmically with the grid current, said plate andscreen grid being passive circuit elements, voltage-responsive measuringmeans connected between said grid and cathode independently of saidplate and responsive to the gridcathode voltage, biasing circuit meansdirectly connected to said plate and filament and biasing said platewith a substantially fixed potential which is positive with respect tosaid control grid and said cathode, and means directly connecting saidscreen grid to said plate to maintain said screen grid at the samepotential as said plate.

5. In an electrical circuit, the combination as defined in claim 4wherein said biasing circuit means biases said plate substantially inthe range of about 1.5 to about 3 volts positive with respect to saidcathode.

6. In an electrical circuit for providing a voltage which varieslogarithmically with variations of a small current and including anelectrometer tube functioning as a nonlinear circuit element and havinga plate, control grid, and a filamentary cathode with the grid andcathode connected into said circuit to provide a logarithmic diode withsaid plate functioning as a passive circuit element, said small currentbeing the grid current of said tube, biasing means directly connected tosaid plate to maintain said plate at a substantially fixed potentialwhich is positive with respect to said cathode in the range of fromabout 1.5 volts to about 3 volts positive with respect to said cathode.

7. In an electrical circuit for providing a voltage which varieslogarithmically with variations of a small current, an electrometer tubefunctioning as a nonlinear circuit element having a plate, a grid, and adirectly heated cathode, input circuit means conducting a small currentto the grid-cathode path of said tube to establish a grid current fromthe grid to the cathode with the grid-tocathode voltage varyinglogarithmically, voltage-responsive output circuit means connectedbetween said grid and cathode, a direct current power supply for heatingsaid cathode, an impedance connecting the positive side of said powersupply to said cathode, and means directly connecting said plate to thepositive side of said impedance so that the potential applied to theplate has a substantially fixed value, said plate being about 1.5 voltsto 3 volts positive with respect to said cathode, said input circuitmeans and said output circuit means being connected independently or"said plate whereby said plate is a passive circuit element.

8. The method of operating a vacuum tube device having a plate, controlgrid and cathode as a logarithmic diode which comprises establishing asmall current to be measured between the grid and cathode of the tube tocause the grid-to-cathode voltage to vary logarithmically with the smallcurrent, and using the plate of the tube as a passive circuit elementwhich is independent of the circuit into which the tube is connected asa nonlinear circuit element and biasing said plate positive by about 1.5to 3 volts with respect to said cathode.

9. In an electrical circuit for measuring a small current, a voltageresponsive amplifier having input terminals, an output circuit and afeedback circuit connected between the input terminals and the outputcircuit, said feedback circuit including a vacuum tube functioning as anonlinear circuit element and having a plate, a grid, and a cathode, thegrid-cathode path of said tube being connected in series in saidfeedback circuit to establish a variable grid current from the grid tothe cathode, said grid and cathode acting as a logarithmic diode inwhich the voltage between said grid and cathode varies as a logarithmicfunction of said grid current, and biasing circuit means connecteddirectly to said plate and biasing said plate with a substantially fixedpotential which is positive with respect to said cathode, said feedbackcircuit being connected independently of said plate whereby said platefunctions as a passive circuit element.

References Cited in the file of this patent UNITED STATES PATENTS2,861,182 Green Nov. 18, 1958 3,058,057 Frost Oct. 9, 1962 FOREIGNPATENTS 936,997 Germany Dec. 1, 1955

1. IN AN ELECTRICAL CIRCUIT FOR PROVIDING A VOLTAGE WHICH VARIESLOGARITHMICALLY WITH VARIATIONS OF A SMALL CURRENT AND INCLUDING AVACUUM TUBE FUNCTIONING AS A NONLINEAR CIRCUIT ELEMENT AND HAVING APLATE, A GRID, AND A CATHODE, INPUT CIRCUIT MEANS CONNECTED TO THEGRID-CATHODE PATH OF SAID TUBE TO ESTABLISH A VARIABLE GRID CURRENT FROMTHE GRID TO THE CATHODE, WHEREBY SAID GRID AND CATHODE ACT AS ALOGARITHMIC DIODE IN WHICH THE VOLTAGE BETWEEN SAID GRID AND CATHODEVARIES AS A LOGARITHMIC FUNCTION OF SAID CURRENT BETWEEN SAID GRID ANDCATHODE, OUTPUT CIRCUIT MEANS CONNECTED BETWEEN SAID GRID AND CATHODEAND RESPONSIVE TO THE GRID-TO-CATHODE VOLTAGE, AND BIASING CIRCUIT MEANSCONNECTING SAID PLATE AND CATHODE AND BIASING SAID PLATE BY DIRECTLYCONNECTING SAID PLATE TO A SUBSTANTIALLY FIXED POTENTIAL WHICH ISPOSITIVE WITH RESPECT TO SAID CATHODE, SAID OUTPUT CIRCUIT MEANS BEINGCONNECTED INDEPENDENTLY OF SAID PLATE, SAID PLATE FUNCTIONING AS APASSIVE CIRCUIT ELEMENT.